Lack of quality spoiled 10 pens (9)
I believe the answer is:
Here is my best explanation:
'lack of quality' is the definition.
'cheapness' can be an answer for 'quality' (cheapness is a kind of quality). I am not sure about the 'lack of' bit.
'spoiled 10 pens' is the subsidiary indication.
I cannot quite understand how this works, but
an anagram of 'pens' is 'pnes' which is present in the answer.
This explanation may well be incorrect...
(Other definitions for cheapness that I've seen before include "Scan sheep (anag)" and "Poor quality".)